DESCRIPTION OF TECHNIQUE

Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a technique used in conjunction with chemical microanalysis by scanning electron microscopy (SEM). (See Manual SEM.) EDS technique detects X-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the volume analyzed. Functions or steps as small as 1 micron or less can be analyzed.

When the sample is bombarded by the electron beam SEM, the electrons are ejected from atoms comprising the sample surface. The resulting electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the states the two electrons’. The X-ray energy is characteristic of the element from which it was issued.

EDS x-ray detector measures the relative abundance of X-rays against their energy. The detector is typically a lithium-drifted silicon solid-state device. When an incident x-ray hits the detector, which creates a pulse of charge that is proportional to the energy of x-ray. The pulse charge is converted to a pulse voltage (which is proportional to the energy xray) by a charge sensitive preamplifier. The signal is then sent to a multichannel analyzer where the pulses are sorted by the tension. The energy, as determined by measuring the voltage per incident x-ray is sent to a computer for display and further data evaluation. The spectrum of x-ray energy versus counts is evaluated to determine the elemental composition of the sample volume.

INFORMATION ANALYSIS

Qualitative analysis – The sample values from x-ray energy spectrum EDS are compared with known characteristic x-ray energy values to determine the presence of an element in the sample. Elements with atomic numbers ranging from beryllium to uranium can be detected. The lower limits of detection range from about 0.1 % to one atom of a few percentage points, depending on the element and the sample matrix.

The quantitative analysis – quantitative results can be obtained from the count on the x-ray energy levels characteristic for components of the sample. Semi-quantitative results are readily available, without using the standard mathematical corrections based on the analysis parameters and the composition of the sample. The accuracy of the standardless analysis depends on the composition of the sample. Greater accuracy is obtained using known standard structure and composition similar to that of the unknown sample.

Elemental Mapping – Characteristic x-ray intensity is measured relative to the lateral position on the sample. Variations in intensity of x-ray, in any characteristic value indicates the relative concentration of energy for the item applicable throughout the area. One or more maps are recorded simultaneously with intensity of brightness as a function of local concentration on the item (s) present. About 1 ìm lateral resolution is possible.

Line Profile Analysis – The SEM electron beam is scanned along a line through the pre-selected sample, while the X-rays are collected for the discrete locations along the line. Analysis of x-ray energy spectrum at each position provides plots of relative concentration for each element for primary position along the line.

TYPICAL APPLICATIONS

* Analysis of foreign materials
* Evaluation of corrosion
* Analysis of the composition of coating
* The rapid identification of the material alloy
* Analysis of material small items
* Phase identification and distribution

SAMPLE REQUIREMENTS

Samples up to 8 inches (200 mm) in diameter, can be easily analyzed in SEM. Larger samples, up to about 12 inches (300 mm) in diameter, can be loaded with the stage movement limited. The maximum height of the sample of about 2 inches (50 mm) can be accepted. The samples should also be compatible with a moderate vacuum environment (pressure of 2 Torr or less)

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